Heretofore, fiber mat boards such as asbestos boards, glass wool boards, etc. are used as sound absorbing boards provided on ceilings, wall surfaces, and so on. These fiber mat boards have a disadvantage that these boards are inferior in sound absorbing property in a low frequency band, while they have a sound absorbing effect in middle and high tone bands.
Therefore, an arrangement of a sound absorbing board in which a porous structure of a flat board with a large number of through-holes formed therein is provided in the front of an air chamber is known as a sound absorbing structure excellent in sound absorbing property in a low frequency band.
However, the conventional porous structure sound absorbing board has a problem as follows. Since the board has a shape in which a large number of holes which open in a plane are formed, these holes are visible from the outside when the board is disposed to a ceiling or wall surface so that a visual point of a person cannot come on a wall surface and an unpleasant feeling is given to the person by flicker. Furthermore, there is a limitation in design because the structure has such a simple shape in which only a large number of holes are formed to open in a plane. Furthermore, there is another problem that the large number of holes are stained to make the external appearance poor or to make the sound absorbing property deteriorate.
The present invention has an object to provide a sound absorbing body having a porous structure which is disposed in the front of an air chamber so as to form a sound absorbing structure with a good sound absorbing property, and which can provide a good external appearance without causing visual flicker.
Another object of the present invention is to provide a sound absorbing board in which while a porous structure is provided with a large number of through-holes, those holes can be prevented from being stained or choked.